numerical study on droplet across micro pillars

8/19/2019 Numerical Study on Droplet Across Micro Pillars

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Numerical study ondroplet sliding across

micropillarsYuxiang Wang and Shuo Chen*

Langmuir 2015, 1, !"#$!"##



%ntroduction and &e'ie( o) rele'ant

literature

&apid progress in research on droplet (etting dynamics due to the promising applications o)

superhydrophobic surfaces in sel) cleaning sur)aces and anti icing o) airplane (ings and po(er lines

euler, + -. /areth, . &oert, C xploiting 3opographical 3exture 3o %mpart %cephoicity +CS Nano

2010, !,#0!4$#052

ishcheno, L. atton, 6. 6ahadur, 7. 3aylor, - + . 8rupenin, 3. +i9energ, - :esign o) %ce;)ree

Nanostructured Sur)aces 6ased on &epulsion o) %mpacting Water :roplets +CS Nano 2010, !, #"<<$##0#

3(o ma=or theories on droplet (etting o) rough sur)aces: Wenzel’s 3heory and Cassie-Baxter’s

3heory 6oth these theories tae area underneath droplet into account maing (ettaility a contact

area problem i.e a 2D problem. Wen9el, & N &esistance o) Solid Sur)aces to Wetting y Water %nd ng Chem 1<", 24, <44$<<

Cassie, + 6 :. 6axter, S Wettaility o) >orous Sur)aces 3rans ?araday Soc 1<!!, !0, 5!"$551

o(e'er there has een heated discussion on (hether droplet (etting is actually a contact line or

contact area prolem 3he deate is ongoing

@11A /ao, L. cCarthy, 3 - o( Wen9el and Cassie Were Wrong Langmuir 200#, 2, #"2$#"5

cale, / Cassie and Wen9elB Were 3hey &eally So Wrong Langmuir 200#, 2, 4200$4205

@1"A 8(on, Y. Choi, S. +nanthara=u, N. Lee, -. >anchagnula, 7. >atanar, N + %s the Cassie;6axter ?ormula&ele'ant Langmuir 2010, 2", 1#524$1#51



3he dynamic properties o) droplet sliding on heterogeneous sur)aces has een studied intensi'ely

ecause it is a ma=or scienti)ic challenge and is particularly use)ul in droplet;ased micro)luidics

Yao, D. u, Y. /rinthal, +. Wong, 3 S. ahade'an, L. +i9energ, - +dapti'e )luid;in)used porous )ilms (ith

tuneale transparency and (ettaility Nat ater 201, 12, 52<$5!

7aragnolo, S. ?erraro, :. ?antinel, >. >ierno, . istura, /.+mati, /. 6i)erale, L. Sragaglia, Stic;slip sliding

o) (ater drops on chemically heterogeneous sur)aces >hys &e' Lett 201, 111, 0""101

En studying the sliding eha'iour o) droplets on sur)aces, and using a particle tracing elocimetry

method to oser'e the )lo( pattern in the sliding droplet directly %n a superhydrophobic surface thereare almost no rolling styles ut )or a hydrophoic sur)aces at constant 'elocity rolling is seen

Su9ui, S. Naa=ima, +. 3anaa, 8. Saai, . ashimoto, +. Yoshida, N. 8ameshima, Y. Eada, 8 Sliding

eha'iour o) (ater droplets on line;patterned hydrophoic sur)aces +ppl Sur) Sci 2004, 25!, 1#<#

ethodology adopted

3his article )ocuses on droplets sliding across pillars (ith 'aried (ettaility on a smooth sustrate, y

using simulation namely !D"D @many;ody dissipati'e particle dynamicsA 3he shape change o) the

droplet (ere oser'ed, and the in)luences on the dynamic contact angles in di))erent stages due to

heterogeneous areas (ere analysed

oogerrugge, > -. 8oelman, - 7 + Simulating icroscopic ydrodynamic >henomena (ith :issipati'e >article

:ynamics urophys Lett 1<<2, 1<, 155$1"0

/root, & :. Warren, > 6 :issipati'e particle :ynamicsBridging the gap et(een atomistic and mesoscopic

simulation - Chem >hys 1<<#, 10#, !!2$!!5

Wang, Y. Chen, S :roplets impact on textured sur)acesB esoscopic simulation o) spreading dynamics +ppl Sur)

Sci 2015,2#, 15<$1"#

?urthermore, the simulated data (as used to illustrate (hy the #ettability is a $D issue.



&esults and :iscussion

%hree different types o) materials (ere used )or the simulation ha'ing different intrinsic contact angles

3hese materials (ere used to create the sustrate and the micropillars 3he droplet (as dri'en y a hori9ontal

ody )orce to slide on the sur)ace 3he ad'ancing and receding contact angles (ere noted and the

displacement o) the droplet )ront (as recorded in the process

3he (hole sliding process can e di'ided into fie stages clearly as seen ao'e

?rom the oser'ed data it is seen that wettability is more sensitive to contact line problem than to a contact

area 3his is illustrated asB

6oth the tale and )igure can 'eri)y that the heterogeneous areas do hae a significant effect (hen the

droplets slide across the pillars in stage as droplet taes more time crossing !& pillar than C pillar

6y Wen9elFs theory or Cassie$6axterFs theory one may expect the same effect in stage ', in (hich the#hole heterogeneous area is in'ol'ed eneath the droplet Gn)ortunately, in stage !, (&' and ()' almost

e*ual (&2 and ()2, though the middle o) liHuidIsolid contact area, (hich is a little )ar a(ay )rom the

contact line, is composed o) a smooth sustrate and pillars (ith t(o di))erent materials Enly the )ront

and rear o) the droplet are on the sustrate as in stage 2

3he droplet 'elocity is lo#er in stage + than in stages 2 and ! oreo'er, in stages 2 and ' the moing elocitiesare almost e*ual 3his )act indicates that in stage ! the pillars do not influence the sliding 'elocity as they do instage %t can e explained y the liHuid particle mo'ing pattern along the contact line 3he inter)acial )luidparticles (hich are far a#ay )rom the three;phase contact line do not moe #ith the droplet Enly the liHuid

particles (hich participate in #etting and de#etting the solid surface near the contact line moe.



/raphs and 3ales

Snapshots o) thesliding o)droplet o'er pillar o)t(o di))erentmaterials sho(ing time taentocross pillar +is morethan thattocross pillar C as +is)ar morehydrophoic 3alesho(s numerso)pillars'ersus timedurationin Stage

, !& !B

1 "40 !!

15"! 4"0



>ersonal Epinions and Comments on the

>aper

3he expository style o) the paper (as lucid and crisp ?acts (ere presented ina clear manner that (as easy to understand and loo up i) needed

3he literature re)erenced (as comprehensi'e and allo(s the reader to getsu))icient acground e)ore reading the paper

+ simple droplet simulation (as used to sho( the 1: nature o) the(ettaility 3he results (ere con'incingly aced up the )indings (hich does

not lea'e anything open to interpretation and clearly points to(ards contactline eing the dominant )actor in droplet (etting

3he data (as clearly presented in the )orm o) graphs (hich allo(s direct'isuali9ation o) the oser'ations

numerical study on droplet across micro pillars

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